Thin electrolyte films can be used in various devices which are required to have low film resistance and high mechanical strength, such as fuel cells, salt electrolysis devices, primary and secondary batteries, membranes for facilitated transport, electrochromic devices, sensors, etc. In particular, they can be used as solid polymer electrolytes for lithium secondary batteries.
Development of the lithium secondary battery of solid polymer electrolyte has been attracting much attention, because of its various advantages, such as controlled formation of dendrite of metallic lithium which may cause damages resulting from short circuit and ignition, no leakage of liquid unlike the case of a solution type secondary battery, and particularly ability of being made into a thin film and large area.
Some of the polymer type solid electrolytes developed so far use a lithium salt such as LiClO.sub.4 dissolved and dispersed in a polymer, such as polyether including polyethylene oxide and polypropylene oxide, polyester, polyimide and polyether derivatives. Such an electrolyte, however, needs a sufficiently higher temperature above room temperature to exhibit an ionic conductivity of 10.sup.-5 to 10.sup.-3 S/cm.
One of the methods to decrease effective resistance of a thin film is to immobilize a liquid ionic conductor by capillary condensation in fine pores, 0.1 .mu.m or less in size, in a thin porous film of a solid polymer, 50 .mu.m or less in thickness, as disclosed by Japanese Patent Laid-Open No. 1-158051. This method, however, does not drastically solve the problems associated with operating temperature.
Some polymer batteries use gelled polymer electrolytes, where the polymer matrices are impregnated with a solution similar to that for the conventional solution type lithium battery. These electrolytes include cross-linked polyalkylene oxide as disclosed by U.S. Pat. No. 4,303,748 and gelled polyacrylate as disclosed by U.S. Pat. No. 4,830,939. These electrolytes, however, still have problems of solvent maintainability, because the electrolytic solution may ooze out at high temperature as a result of gel shrinkage.
It is well known that some polymers are incorporated with an electron-conductive substance, such as carbon black, to be made electroconductive. Such polymers include polyethylene, polypropylene, polyvinyl chloride, polysulfone, butadiene rubber, silicone rubber, ethylene-propylene-diene terpolymers. These electroconductive polymers are used for anti-static devices, electromagnetic wave shielding devices, electroconductive paints, adhesives, IC packaging materials, and planar heating sheets and switches.
For the conductor, a thin-film conductor having a high conductivity can be effectively used as the electrode or as a material for the electrode of the device which involves a solid polymer or liquid electrolyte. It provides a large contact interface between the electrode and electrolyte, making it suitable for primary and secondary lithium batteries of high functions.
Japanese Patent Laid-Open No.3-87096 discloses a porous electroconductive film and production thereof, where an electrolytic solution is immobilized by capillary condensation force on the porous thin film produced from a plasticizer solution of polyethylene incorporated with Keten black (trade name of Akzo Chemicals), which is molded into sheet, drawn and treated to remove the plasticizer. However, the problems associated with maintainability of electrolytic solution are not completely solved by this technique. More recently, a new technique has been developed that uses a polymer gel as the anode or cathode of the battery, where the polymer gel is impregnated with a carbonate-based solution with LiMn.sub.2 O.sub.4 and carbon black or petroleum coke and carbon black dissolved in a copolymer of polyvinylidene fluoride and hexafluoropropylene to dissolve a lithium salt, as disclosed by U.S. Pat. No. 5,296,318. These electrolytes, however, have still problems of solvent maintainability, because the electrolytic solution may ooze out at high temperature as a result of gel shrinkage. Therefore, thin-film conductors which can be easily produced to have a large area and exhibit stable maintainability of the electrolytic solution over a wide temperature range are increasingly demanded.
It is an object of the present invention to solve the above problems, and to provide a thin film of non-protonic electrolyte which is easily produced into a thin film and to have a large area, holds the solvent for the non-protonic electrolytic solution over a wide temperature range, is stable over extended periods and has improved mechanical strength; an electrolyte-immobilized liquid-film conductor; and a method of production thereof.